Ruling is not easy.
“Because vassals only provide a percentage of their military strength to their liege but will field all of their strength in the event they rebel (typically through forming a faction whose demands are refused), large realms can be dangerously unstable unless the ruler can maintain a large personal territory (the demesne) from which they can draw the backbone of their own army independent of their vassals; doing so under partible inheritance is particularly hard (as you can see concerns about keeping the royal possessions together dominating a lot of my House al-Yiliq strategy). “